1. Field of the Invention
The present invention relates to primer compositions, and more particularly to a polyorganosiloxane primer composition for use in the coating of paints on the surfaces of crosslinked polyorganosiloxanes, and especially on the surfaces of crosslinked and cured silicone rubbers.
2. Description of The Prior Art
Silicone rubber which has been cured by crosslinking, for example, silicone rubber sealants and coating materials, has an excellent heat resistance, cold resistance, weather resistance and fatigue resistance and so has come to be employed in a broad range of applications. In particular, silicone sealants are advantageously applied in joints susceptible to considerable movement or at outdoor locations.
The cured form of these silicone rubber sealants and coatings suffer from the significant defect that their surfaces cannot be adequately coated with paint, or the coating operation is a difficult one. Due to this defect, silicone rubber sealants and coatings are limited in their applications, although they have excellent properties.
Various methods have therefore been considered for improving the paintability of cured silicone rubber sealants. One such method involves the use of additives, and is taught in Japanese Laid-Open Patent Application Number 58-157860 [157860/83] and in U.S. Pat. No. 4,447,576, which issued to Fukayama and Ichiijo on May 8, 1984. Both of these publications describe curable organosiloxane compositions comprising a hydroxyl terminated polyorganosiloxane and a silicone compound containing two or more amide and/or aminoxy groups. The additive present in the compositions of the Japanese application is a dialkoxysilane having a mercapto, amino, acryloxy or epoxy group that is bonded to silicon through a divalent organic group. The additive in the compositions of the United States Patent is an alcohol with an acyloxy, alkoxycarbonyl, --COOH, cyano, amino, amido or mercapto group or the combination of a carbon-to-carbon triple bond and at least one ether type oxygen atom. These additives are less than desirable because they are limited to a specific curing system and therefore cannot be used with all types of curable silicone rubber compositions. Another problem with these additives is that they detract from the unmodified properties of the cured silicone rubber to a greater or lesser degree.
Primer compositions have also been investigated as an alternative approach to improving the paintability of cured silicone rubber. For example, Canadian Pat. No. 1,142,818, which issued on Mar. 15, 1983 teaches applying an amino substituted silane as a primer prior to coating cured silicone rubber with paint. However, when only this amino group-containing alkoxysilane is employed as the primer between silicone rubber and paint, the bonding strength is poor. In particular, amino group- containing dialkoxysilanes provide little bonding strength.
Japanese published application No. 55-62,960 [62,960/80], published on May 12, 1980, discloses primer compositions for bonding cold-curing silicone rubber to plastics and rubbers. The compositions contain 100 parts by weight of an essentially linear polydiorganosiloxane containing at least two silicon-bonded hydroxyl groups per molecule, from 1 to 100 parts by weight of a silane containing at least 2 alkoxy groups and an aminoalkyl group bonded to silicon, and from 1 to 100 parts by weight of ethyl silicate or a partial hydrolyzate thereof.
The object of the present invention is to provide a primer composition which (1) will not adversely affect the properties of the cured silicone rubber sealant or coating, (2) can be used with any silicone rubber curing system, and (3) can tightly bond paint to the cured silicone rubber.
The present inventors have discovered that this objective can be achieved by omitting the ethyl silicate or a hydrozylate thereof from compositions described in the aforementioned Japanese published application No. 55-62,960, and using the combination of an amino substituted dialkoxysilane, referred to hereinafter as ingredient A, with a liquid polyorganosiloxane containing at least two hydroxyl groups per molecule, referred to hereinafter as ingredient B.